In recent years, music distribution service to distribute music data via the Internet or the like has been spreading. With this music distribution service, encoded data obtained by encoding music signals is distributed as music data. As a music signal encoding technique, an encoding technique has become the mainstream wherein a bit rate is lowered while suppressing file capacity of encoded data so as not to take time at the time of downloading.
Such a music signal encoding techniques, are roughly divided into an encoding technique such as MP3 (MPEG (Moving Picture Experts Group) Audio Layer 3) (International Standards ISO/IEC 11172-3) and so forth, and an encoding technique such as HE-AAC (High Efficiency MPEG4 AAC) (International Standards ISO/IEC 14496-3) and so forth.
With the encoding technique represented by MP3, of music signals, signal components in a high-frequency band (hereinafter, referred to as high-frequency) equal to or greater than around 15 kHz of hardly sensed by the human ear, are deleted, and signal components in the remaining low frequency band (hereinafter, referred to as low-frequency) are encoded. Such an encoding technique will be referred to as high-frequency deletion encoding technique. With this high-frequency deletion encoding technique, file capacity of encoded data may be suppressed. However, high-frequency sound may slightly be sensed by the human ear, and accordingly, at the time of generating and outputting sound from music signals after decoding obtained by decoding encoded data, there may be deterioration in sound quality such as loss of sense of presence that the original sound has, or the sound may seem to be muffled.
On the other hand, with the encoding technique represented by HE-AAC, characteristic information is extracted from high-frequency signal components, and encoded along with low-frequency signal components. Herein after, such an encoding technique will be referred to as a high-frequency characteristic encoding technique. With this high-frequency characteristic encoding technique, only characteristic information of high-frequency signal components is encoded as information relating to the high-frequency signal components, and accordingly, encoding efficiency may be improved while suppressing deterioration in sound quality.
With decoding of encoded data encoded by this high-frequency characteristic encoding technique, low-frequency signal components and characteristic information are decoded, and high-frequency signal components are generated from the low-frequency signal components and characteristic information after decoding. Thus, a technique to expand the frequency band of low-frequency signal components by generating high-frequency signal components from low-frequency signal components will hereinafter be referred to as a band expanding technique.
As one application of the band expanding technique, there is post-processing after decoding of encoded data by the above-mentioned high-frequency deletion encoding technique. With this post-processing, high-frequency signal components lost by encoding are generated from the low-frequency signal components after decoding, thereby expanding the frequency band of the low-frequency signal components (see PTL 1). Note that the frequency band expanding technique according to PTL 1 will hereinafter be referred to as the band expanding technique according to PTL 1.
With the band expanding technique according to PTL 1, a device takes low-frequency signal components after decoding as an input signal, estimates high-frequency power spectrum (hereinafter, referred to as high-frequency frequency envelopment, as appropriate) from the power spectrum of the input signals, and generates high-frequency signal components having the high-frequency frequency envelopment from the low-frequency signal components.
FIG. 1 illustrates an example of the low-frequency power spectrum after decoding, serving as the input signal, and the estimated high-frequency frequency envelopment.
In FIG. 1, the vertical axis indicates power by a logarithm, and the horizontal axis indicates frequencies.
The device determines the band of low-frequency end of high-frequency signal components (hereinafter, referred to as expanding start band) from information of the type of an encoding method relating to the input signal, sampling rate, bit rate, and so forth (hereinafter, referred to as side information). Next, the device divides the input signal serving as low-frequency signal components into multiple subband signals. The device obtains average for each group regarding a temporal direction of power (hereinafter, referred to as group power) of each of multiple subband signals following division, that is to say, the multiple subband signals on the lower frequency side than the expanding start band (hereinafter, simply referred to as low-frequency side). As illustrated in FIG. 1, the device takes a point with average of group power of each of the multiple subband signals on the low-frequency side as power, and also the frequency of the lower end of the expanding start band as the frequency, as the origin. The device performs estimation with a primary straight line having predetermined inclination passing through the origin thereof as frequency envelopment on higher frequency side than the expanding start band (hereinafter, simply referred to as high-frequency side). Note that a position regarding the power direction of the origin may be adjusted by a user. The device generates each of the multiple subband signals on the high-frequency side from the multiple subband signals on the low-frequency side so as to obtain the estimated frequency envelopment on the high-frequency side. The device adds the generated multiple subband signals on the high-frequency side to obtain high-frequency signal components, and further adds the low-frequency signal components thereto and output these. Thus, music signals after expanding the frequency band approximates to the original music signals. Accordingly, music signals with high sound quality may be played.
The above-mentioned band expanding technique according to PTL 1 has a feature wherein, with regard to various high-frequency deletion encoding techniques and encoded data with various bit rates, the frequency band regarding music signals after decoding of the encoded data thereof can be expanded.